PSA: Don't update to iOS 12.4 if you use apps in AB output slot or IAA apps that use the mic.
It may be too late for some of you, but I've just discovered a late-breaking issue with iOS 12.4 which seems to affect at least some output apps and their ability to receive audio through Audiobus.
Still investigating, but for those of you lucky enough to still be on pre-12.4, I urge you to hold off updating for now. Will update this thread with details as they emerge.
Update: I've applied an emergency stop-gap, which forces manual "Tap to launch" launching of all apps that can be put in the audio output slot in Audiobus. This seems to resolve the issue for now, although it's a little inconvenient.
The stop-gap has been applied immediately, as it's a modification of the Audiobus database, so it will work right away.
Update: @Janosax has discovered downgrading is still possible if you've already updated. See below. This ability won't last long though.